1. Field of the Invention
The present invention relates to a process for producing a cold rolled steel strip highly susceptible to conversion treatment. More particularly, the present invention relates to a process for enhancing a conversion treatment property of at least one non-plated surface of a cold rolled steel strip. In that, the cold rolled steel strip produced in accordance with the present invention provides at least one non-plated surface thereof exhibiting an enhanced conversion treatment property, for example, an enhanced phosphate-coating property and lacquering property. The phosphate-coated, laquered surface exhibits an excellent resistance to corrosion.
2. Description of the Prior Art
Usually, a cold rolled steel strip is produced by descaling a hot rolled steel strip by means of pickling, and by then cold rolling the descaled hot rolled steel strip. In order to enhance certain properties, for example, the phosphate-coating property and lacquering property, of the cold rolled steel strip, the cold rolled steel strip is surface-cleaned, for example, by means of an electrolytic degreasing method, and is then introduced into a batch type box-annealing furnace. In the furnace, the degreased steel strip is heated to a recrystallization temperature of the steel strip or more, is soaked at the above-mentioned temperature, and is then cooled in a reducing gas atmosphere to a temperature at which the steel strip surface is not oxidized. The cooled steel strip is removed from the annealing furnace and is additionally cooled to a temperature at which steel strip is not aged. The cooled steel strip is then subjected to a temper rolling procedure.
The above-mentioned conventional process is disadvantageous in productivity and in economical efficiency not only in that the process includes a number of steps and therefore, complicates the handling of the connections between the steps, but also in that since the steel strip is coiled in the box-annealing furnace and the coil is subjected to the heating, soaking, and cooling steps, a long period of time is necessary to complete the annealing procedure.
Accordingly, it is desired that above-mentioned steps after the cold rolling step be made concise and continuous and that the productivity and economical efficiency of the steps be improved.
In recent years, various approaches have been tried for making the above-mentioned annealing steps continuous so as to produce a cold rolled steel strip having an enhanced workability at a high economical efficiency. In these approaches, a cold rolled steel strip is heated at a recrystallizing temperature thereof or more, is primarily cooled to a predetermined temperature, is then overaged at a predetermined temperature for a predetermined time, and finally, is secondarily cooled to a room temperature, so as to control the thermal history of the steel strip to a predetermined pattern thereof.
Generally, it is possible to produce a cold rolled steel strip at a high efficiency by using a continuous annealing process. However, the conventional continuous annealing process is disadvantageous in that even if the annealing procedure is continuously carried out in a reducing gas atmosphere by heating a steel strip by means of a heat-radiation tube type continuous annealing furnace, and by cooling it by means of a cooling jet, the phosphate-coating property of the resultant steel strip is not so good as that of the steel strip annealed by means of a batch type box annealing furnace.
Especially, when the annealing process is carried out by a combination of a rapid heating operation by means of a direct heating furnace with a rapid cooling operation by means of a cooling medium consisting of a gas and water or of cooling water, the resultant steel strip exhibits an unsatisfactory phosphate-coating property. The direct heating furnace heating operation and the gas-water cooling or water cooling operation are carried out substantially in an oxidizing atmosphere, and therefore, the surface of the steel strip is oxidized in the heating operation and in the cooling operation.
Accordingly, it is necessary that in a certain stage of the continuous annealing step, the steel strip is subjected to a step in which the resultant layer of oxides is removed from the steel strip surface. However, it should be noted that even when the oxide layer produced in the direct heating furnace can be reduced in a soaking furnace at an elevated temperature, the reduced surface of the steel strip is re-oxidized in the cooling step and the resultant oxide layer cannot be reduced in the overaging step, which is carried out at relatively low temperature. Therefore, it is difficult to shorten the continuous annealing process. Also, if the reduction of the oxide layer is carried out incompletely, the resultant steel strip surface exhibits an unsatisfactory phosphate-coating property and, therefore, an unsatisfactory lacquering property, and the resultant lacquered steel strip exhibits a poor resistance to corrosion. Therefore, it is necessary that before the temper rolling process, the oxide layer is completely removed by means of pickling, abrading or grinding. These procedures cause the phosphate-coating property of the steel strip to decrease.
As a recent trend, the steel strip used for the body of car is usually a single surface plated steel strip. That is, the plated surface of the steel strip is utilized for forming portions of the surface of the car body which are not lacquered, for example, the inside surface of the core, and the non-plated surface of the steel strip is utilized to form the other portions of the car body surface, for example, the outside surface thereof, which are easily lacquered. The single surface-plated steel strip is produced by plating a single surface of a steel strip with a zinc-based alloy by means of a hot valcanizing or electroplating method. Usually, the electroplating method is used for the production of the single surface-plated steel strip, because in the electroplating method the steel strip can be processed various ways.
In the production of the single surface-plated steel strip, a steel strip is immersed in a plating liquid and is placed between an upper electrode and a lower electrode. When an electric current is applied between the steel strip and the lower electrode and no current is applied between the steel strip and the upper electrode, only the lower surface of the steel strip is plated and upper surface of the steel strip is retained as non-plated. However, in the above-mentioned single surface-plate method, the non-plated upper surface of the steel strip is undesirably polluted with a small amount of plating metal deposited thereon. Also, in the water-rinsing, hot water-rinsing, and drying steps, the upper surface of the steel strip is polluted with oxides or hydroxides. Usually, the small amount of plating metal deposited on the non-plated surface is in the amorphous or semi-amorphous state. Therefore, when a conversion treatment is applied to the polluted non-plated surface of the steel strip, the plating metal layer hinders the formation of a regular coating layer and causes undesirable coating defects to be formed.
There are various approaches to the removal of the undesirable deposits from the non-plated surface of the steel strip. For example, a brushing operation is applied to the polluted non-plated surface. However, this operation is unsatisfactory in that it does not completely remove the deposits from the non-plated surface.
In another approach, Japanese Unexamined Patent Publication (Kokai) No. 59-70792 discloses a process for removing the deposits from the non-plated surface of a steel strip by means of an anodic electrolytical treatment in a specific electrolyte solution containig a specific amount of a surface active agent. This anodic electrolytical treatment should be carried out at a neutral pH range of from 4 to 10. If the anodic electrolytical treatment is carreid out in a strong acid range or strong alkaline range of pH, a portion of the iron in the steel strip is dissolved together with the deposits in the electrolytic liquid. This phenomenon results in etching of the non-plated surface of the steel strip and in degradation of the elecyrolytic liquid by the dissolved iron (Fe.sup.++). When the anodic electrolytic treatment is carried out in a neutral pH range, the non-plated surface of the steel strip matrix is covered with a passive state layer, that is, an oxide layer, and therefore, no iron (Fe.sup.++) is dissolved in the electrolytic liquid. That is, no etching of the non-plated surface and substantially no degradation of the electrolytic liquid occurs. That is, when the non-plated surface of the steel strip is subjected to anodic electrolytic treatment, a passive state (oxide) layer is formed on the non-plated surface. Usually, this passive state layer does not obstruct the conversion treatment, for example, phosphate-coating process. However, where a high purity steel strip or a steel strip containing at least one element selected from titanium, niobium and boron is subjected to a conversion treatment, the passive state layer will sometimes obstract the conversion treatment so as that, for example, the formation of the phosphate-coating layer is hindered. Especially, when the conversion treatment is carried out by means of spraying or dipping method, and the conversion treatment liquid is partially degraded, the passive state layer hinders the conversion treatment.
In any type of steel strip, in any type of conversion treatment liquid, and in any type of production line, the conversion coating must be always stably formed.
Japanese Unexamined Patent Publication (Kokai) No. 58-133395 discloses a process for removing black substances consisting of amorphous oxides and hydroxides from the non-plated surface of a steel strip which has been plated on a single surface thereof. In this process, the non-plated surface is subjected to an anodic electrolytic treatment in aqueous solution containing at least one member selected from sulfuric acid, hydrochloric acid, perchloric acid, carobonic acid, boric acid, and nitric acid, and at least one member selected from sodium hydroxide, potassium hydroxide, said perchloric acid, carbonic acid, boric acid, and nitric acid, and at least one member selected from sodium hydroxide, patassium hydroxide, and ammonium hydroxide, at a pH of from 3 to 9 and at an anode current density of 5 A/dm.sup.2. In this anodic electrolytic treatment, a passive state layer (oxide layer) is naturally formed on the non-plated surface of the steel strip and, sometimes, hinders the conversion treatment.